

def is_prime(n):
    for i in range(2, n):
        if n % i == 0:
            return False
    return True
n = input()
if len(n) != 4:
    print(0)
    exit()
num = [int(n[0]), int(n[1]), int(n[2]), int(n[3]), int(n[0:2]), int(n[0:3]), int(n[2:4]), int(n[1:4]), int(n[1: 3]), int(n)]
result = []
for i in num:
    if is_prime(i) == True:
        result.append(i)
if len(result) > 0:
    print(max(result))
else:
    print(0)





































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































